Method and apparatus for determining absorption of electromagnetic radiation by a material

ABSTRACT

A method of determining a portion of light at a given wavelength which is incident on a material that is absorbed by the material, the method comprising: transmitting a pulse of light at the given wavelength so that the pulse traverses a path through the material; generating a first signal responsive to light in the light pulse that traverses the path length without being absorbed by the material; generating a second signal responsive to energy that the material emits responsive to a portion of the light from the light pulse that is absorbed by the material as the light pulse traverses the path; and using the first and second signals to determine the absorbed portion.

RELATED APPLICATIONS

The present application claims benefit under 35 U.S.C. 119(e) of U.S.Provisional application 60/327,288 filed Oct. 9, 2001.

FIELD OF THE INVENTION

The invention relates to determining absorption of electromagneticradiation by a material and in particular absorption of light by amaterial.

BACKGROUND OF THE INVENTION

It is well known to assay components of a material by measuringabsorption coefficients of the material for light at suitablewavelengths. A component of a material contributes to an absorptioncoefficient of the material for light at a given wavelength inproportion to σρ where ρ is a concentration of the component and σ is anabsorption cross-section of the component for light at the givenwavelength.

To determine an absorption coefficient of a material for light at agiven wavelength, generally, a sample of the material is illuminatedwith light at the given wavelength. An amount of the light that istransmitted through the sample is measured to determine attenuation thatthe light suffers in passing through the sample and the Beer-Lambert lawis then used to determine the absorption coefficient. If α representsthe absorption coefficient and L the optical path-length of the lightthrough the material, then by the Beer-Lambert law I=I_(o) exp (−αL),where I is the intensity of light transmitted through the material andI_(o) is the intensity of light incident on the material.

Generally α is a function of concentration of a plurality of differentcomponents in the material. In order to determine concentration of aparticular component of the material, α is measured at a plurality ofdifferent wavelengths. The concentration of a particular component ofthe material is determined from known absorption cross sections ofcomponents of the material and the measurements of the absorptioncoefficient at the different wavelengths. U.S. Pat. No. 5,452,716 to V.Clift, the disclosure of which is incorporated herein by reference,describes measuring absorption coefficients for blood at a plurality ofwavelengths to assay blood glucose.

Numerous devices, hereinafter referred to as “photometers”, of variousdesigns are available for measuring an absorption coefficient of amaterial. The devices comprise a suitable light source, such as a laseror LED, which provides a beam of light that is passed through a sampleof a material to be tested. Intensity of the provided beam of light ismeasured to provide a value of I_(o) and a measurement of intensity oflight transmitted through the sample provides a value for I. A value foran optical path-length L of the beam of light through the sample isgenerally determined from a shape of the sample, or in the case of aliquid, often from a shape of a cuvette that contains the liquid.

In some photometers, referred to as “vertical-beam photometers”, thatare used to determine absorption coefficients of a liquid, a sample ofthe liquid is held in an open receptacle. A beam of light is transmitted“vertically” through the open end of the receptacle, the liquidcontained in the receptacle and the bottom of the receptacle todetermine attenuation of the beam and thereby an absorption coefficientof the liquid. The optical path-length L of the light beam through theliquid is determined by the height to which the receptacle is filledwith the liquid and a shape of a meniscus formed at an interface of theliquid with the air.

In general, both the height of a liquid sample in a receptacle of avertical-beam photometer and the shape of its meniscus cannot becontrolled to an accuracy with which dimensions of a cuvette can becontrolled. As a result, optical path-lengths of light through liquidsamples in a vertical-beam photometer are generally not as accuratelyknown or controllable as optical path-lengths through samples in aphotometer for which optical path-lengths are determined by dimensionsof a cuvette. Measurements of absorption coefficients provided byvertical-beam photometers are therefore generally not as accurate asmeasurements of absorption coefficients provided by other types ofphotometers.

However, vertical-beam photometers are popular because they enable rapidsampling of large numbers of liquid samples. The receptacles that holdliquid samples to be tested are generally formed as small wells in“trays” produced from a suitable material. The wells in a tray areeasily and quickly filled with liquids to be tested. Once filled, thetray is rapidly positioned to expose the liquid in each of the wells toa beam of light that the photometer provides for measuring absorptioncoefficients.

U.S. Pat. No. 6,188,476, the disclosure of which is incorporated hereinby reference, discusses the problem of determining optical path-lengthsof liquid samples whose absorption coefficients are measured usingvertical beam photometry. The patent describes methods for determiningoptical path-lengths of the sample solutions using calibrationmeasurements of path-lengths at two different wavelengths for variouscommon solvents that the liquid samples might contain.

In addition to errors in absorption coefficient measurements generatedby errors in determination of optical path-lengths L, absorptioncoefficient measurements provided by photometers are often subject toerror resulting from variations in intensity of light I_(o) provided bythe light source and drift in sensitivity of a detector used todetermine I.

SUMMARY OF THE INVENTION

An aspect of some embodiments of the present invention relates toproviding an improved photometer for determining an absorptioncoefficient for light of a sample of a material.

An aspect of some embodiments of the present invention relates toproviding a photometer that determines an optical path-length for a beamof light that is transmitted through a sample of a material to determinea value for an absorption coefficient of the material.

An aspect of some embodiments of the present invention relates toproviding a photometer that provides a measurement of an absorptioncoefficient of a sample that is substantially independent of variationsin intensity of a light beam that is transmitted through the sample todetermine the coefficient of absorption.

According to an aspect of some embodiments of the present invention, thevalue of the absorption coefficient is substantially unaffected by driftin sensitivity of a detector used to determine intensity of light in thelight beam that is transmitted through a sample.

A photometer, in accordance with an embodiment of the present inventioncomprises a light source and an energy detector. The energy detectorgenerates a signal responsive to energy incident thereon from whichsignal an amount of the incident energy can be determined.

The energy detector is coupled to a sample of a material for which anabsorption coefficient is to be determined and the light source iscontrolled to provide at least one pulse of light that is transmittedinto the sample. Some of the light in a light pulse that is transmittedinto the sample is absorbed by the material and some of the light in thelight pulse is not absorbed and is transmitted through the material. Thelight source and the energy detector are positioned so that at least aportion of light in the light pulse that is not absorbed by the materialreaches the energy detector as a pulse of optical energy either directlyfrom the light source or by reflection from the material. (i.e. in someembodiments of the present invention, the pulse of optical energyreaches the detector along a direct path through the material from thelight source to the detector. In some embodiments of the presentinvention the pulse of optical energy reaches the detector afterreflection by the material.)

The pulse of optical energy, hereinafter referred to as “immediateenergy”, from the non-absorbed light reaches the detector following agenerally very short delay that is determined by a distance that thelight travels from the light source to the detector divided by the speedof light. In response to the immediate energy, the detector generates asignal, hereinafter referred to as an “immediate signal”, from whichsignal the intensity and amount of immediate energy incident on thedetector are, optionally, determined using methods known in the art.

Energy from light in the light pulse that is absorbed by the material issubsequently released and a portion of the released energy, hereinafterreferred to as “delayed energy”, reaches the energy detector after theimmediate energy reaches the detector. A time period between the arrivalof the immediate energy at the detector and arrival of the delayedenergy at the detector is hereinafter referred to as an “absorptiondelay”. The absorption delay is a sum of a “propagation delay” and a“release delay”. Generally, the delayed energy propagates to the energydetector from a point in the material at which the energy is released asan acoustic wave or as heat propagated by convection. A differencebetween the speed of light and speed of sound or thermal convection inthe material causes the propagation delay. The release delay is a timebetween a time at which light is absorbed by the material and a time atwhich energy absorbed from the light is released by the material.Generally, the propagation delay is much longer than the release delayand the absorption delay is dominated by the propagation delay.

The energy detector generates a signal, hereinafter referred to as a“delayed signal”, in response to an amount of delayed energy thatreaches the detector. As in the case of the immediate signal, intensityand an amount of delayed energy incident on the detector are,optionally, determined from the delayed signal.

In some embodiments of the present invention, the energy detectorcomprises an acoustic detector. Immediate energy reaches the acousticdetector in the form of a pulse of optical energy from light in thelight pulse that is not absorbed by the material and generates anacoustic pulse in the acoustic detector, responsive to which thedetector generates the immediate signal. Light from the light pulse thatis absorbed by the material generates sound waves in the material by aphotoacoustic process. The sound waves propagate to the acousticdetector and transport “delayed energy” to the detector, responsive towhich the detector generates the delayed signal.

Generation of acoustic waves by the photoacoustic effect is discussed inIsrael Patent Application 138,073 entitled “Photoacoustic Assay andImaging System”, filed on Aug. 24, 2000, by some of the same applicantsas the applicant of the present invention and in PCT applicationPCT/IL01/00740, having the same title, both of which disclosures areincorporated herein by reference. The relationship between the amplitudeof a photoacoustic wave and an amount of energy absorbed by a region oftissue that generates the photoacoustic wave is described in U.S. Pat.No. 4,385,634 to Bowen and PCT publication WO 98/14118 the disclosuresof which are incorporated herein by reference. Expressions for amplitudeof a photoacoustic wave are also given in an article by Lai, H. M. andYoung, K. J. in Acoust. Soc. Am. Vol 76, pg 2000 (1982), in an articleby MacKenzie et al., “Advances in Photoacoustic Noninvasive GlucoseTesting”, Clin. Chem. Vol 45, pp 1587-1595 (1999) and in an article byC. G. A. Hoelen et al., “A New Theoretical Approach To PhotoacousticSignal Generation”, Acoust. Soc. Am. 106 2 (1999) the disclosures of allof which are incorporated herein by reference.

The immediate and delayed energies are proportional respectively to theamount of light from the light pulse that is not absorbed by thematerial during transit of the light pulse through the material and theamount of light that is absorbed by the material during transit of thelight pulse through the material. In accordance with an embodiment ofthe present invention, the immediate and delayed signals are processedto provide a ratio, hereinafter an “absorption ratio”, between theamount of light absorbed from the light pulse and the amount of lightthat is not absorbed from the light pulse. Since both the absorbed andnon-absorbed amounts of light are proportional to the intensity of lightin the light pulse, the absorption ratio is substantially independent ofintensity of light in the light pulse. The absorption ratio is afunction substantially only of an absorption coefficient of the materialfor light in the light pulse and a path-length of the light pulsethrough the sample. In accordance with an embodiment of the presentinvention, the absorption ratio is used to determine the absorptioncoefficient.

The absorption ratio is a particularly sensitive measure of theabsorption coefficient since the absorption ratio changes substantiallymore rapidly with a change in absorption coefficient than either theamount of energy absorbed or not absorbed by the material from the lightpulse. The absorption ratio is also substantially independent ofintensity of light in the light pulse. Furthermore, since in accordancean embodiment of the present invention, a same energy detector sensesand generates signals responsive to both the immediate and delayedenergies the absorption ratio is substantially independent of changes insensitivity of the detector. It is noted that in prior art photometerstwo detectors are generally used to determine an absorption coefficientof a sample of a material. One of the detectors measures intensity“I_(o)” of light transmitted by a light source into the sample and asecond detector measures intensity of light “I” that is transmittedthrough the sample. Changes in relative sensitivity of the two detectorsor in an optical system that directs a portion of the light from thelight source to the first detector and a portion to the sample aresources of error that can compromise accuracy of a measurement providedby such a prior art photometer. A photometer, in accordance with thepresent invention is substantially independent of such sources of error.A photometer, in accordance with an embodiment of the present inventiontherefore generally provides a particularly robust and sensitive measureof absorption coefficient.

For embodiments of the present invention for which the detector is anacoustic detector a portion of the acoustic energy repeatedly bouncesback and forth between the detector and a surface of the sample. Thespeed of sound in the sample and frequency with which energy bouncesback and forth between the detector and the surface is used, inaccordance with an embodiment of the invention, to determine a distancebetween the detector and the surface and thereby a path-length for lightthrough the sample. In some embodiments of the present invention, avalue for the speed of sound in the sample used to determine a distancebetween the detector and the surface is experimentally determined from atime it takes for sound to travel a known distance through the sample.For example, if the sample is a liquid contained in a cuvette, the speedof sound can be determined by positioning a suitable acoustic transduceron a side of the cuvette below a level of the liquid in the cuvette. Thetransducer is used to measure a time it takes sound to travel back andforth in the liquid between sides of the cuvette. Since the dimensionsof the cuvette are known, the speed of sound in the liquid can bedetermined.

In some embodiments of the present invention, the detector comprises athermal transducer. Direct energy that reaches the thermal transducerfrom light in the light pulse that is not absorbed by the materialgenerates a change in temperature of the thermal transducer, responsiveto which change in temperature the detector generates the immediatesignal. The detector generates the delayed signal responsive to thermalenergy that reaches the detector, which is released by the materialresponsive to light absorbed by the material from the light pulse.

There is therefore provided in accordance with an embodiment of thepresent invention a method of determining a portion of light at a givenwavelength which is incident on a material that is absorbed by thematerial, the method comprising: transmitting a pulse of light at thegiven wavelength so that the pulse traverses a path through thematerial; generating a first signal responsive to light in the lightpulse that traverses the path length without being absorbed by thematerial; generating a second signal responsive to energy that thematerial emits responsive to a portion of the light from the light pulsethat is absorbed by the material as the light pulse traverses the path;and using the first and second signals to determine the absorbedportion.

Optionally, the method comprises determining a path length for the paththat the light pulse traverses and using the determined path length andthe absorbed portion to determine an absorption coefficient of thematerial for the given wavelength.

Optionally, the energy that the material emits comprises a pulse ofacoustic energy generated in the material by a photoacoustic effect andgenerating the second signal comprises sensing the acoustic energy andgenerating a signal responsive thereto.

Optionally, the path through the material is bounded by two surfaces anda portion of the energy in the photoacoustic pulse emitted by thematerial repeatedly bounces back and forth between the two surfaces anddetermining a path length for the path comprises determining a timeperiod it takes for energy in the acoustic pulse to make a round tripbetween the surfaces and using the time period to determine the pathlength.

Alternatively or additionally the energy that the material emitscomprises a pulse of acoustic energy generated in the -material by aphotoacoustic effect and generating the second signal comprises sensingthe acoustic energy and generating a signal responsive thereto.

In some embodiments of the present invention, the energy that thematerial emits comprises thermal energy and generating the second signalcomprises sensing the thermal energy and generating a signal responsivethereto.

In some embodiments of the present invention, the energy that thematerial emits comprises optical energy luminesced by the material andgenerating the second signal comprises sensing the luminesced light andgenerating a signal responsive thereto.

In some embodiments of the present invention, generating a first signalcomprises sensing optical energy in the non-absorbed light, transducingthe sensed energy to acoustic energy and generating a signal responsiveto the acoustic energy.

Optionally, sensing optical energy in the non-absorbed light comprisessensing light from the light pulse that is scattered by the materialrelative to a direction of propagation of the at least one light pulse.

In some embodiments of the present invention, generating a first signalcomprises sensing optical energy in the non-absorbed light, transducingoptical energy in the non-absorbed light to thermal energy andgenerating a signal responsive to the thermal energy.

In some embodiments of the present invention, comprising sensing energyoriginating in the light pulse as a function of time followingtransmission of the light pulse through the material and generating thefirst signal comprises generating the first signal responsive to energysensed within a time period after transmission of the light pulse thatis less than or equal to about twice a transit time of light from thelight pulse over the path.

Optionally, generating the second signal comprises generating a secondsignal responsive to energy sensed at time following the light pulsetransmission time that is substantially later than the transit time.

In some embodiments of the present invention, using the first and secondsignals to determine the absorbed portion comprises: using the firstsignal to provide an indication of energy in the light pulse that is notabsorbed by the material; using the second signal to provide anindication of energy in the light pulse that is absorbed by thematerial; and using the indicated energies to determine the absorbedportion.

Optionally, using the indicated energies to determine the absorbedportion comprises determining a quotient between the indicated energies.

There is further provided in accordance with an embodiment of thepresent invention, apparatus for determining an absorption coefficientof a material for light of a given wavelength comprising: a light sourcethat transmits a pulse of light at the given wavelength that traverses apath through the material; a detector that receives light from the lightpulse that is not absorbed by the material and generates a first signalresponsive thereto; a detector that receives energy emitted by thematerial responsive to light from the light pulse that is absorbed bythe material and generates a second signal responsive to the receivedenergy; and a processor that receives the first and second signals anduses the signals to determine the absorption coefficient.

Optionally, the detector that receives light from the light pulsecomprises an acoustic sensor that converts optical energy from the lightpulse incident on the detector to acoustic energy responsive to whichacoustic energy the detector generates the first signal.

Alternatively the detector that receives light from the light pulse isoptionally a thermal sensor that converts optical energy from the lightpulse incident on the detector to thermal energy, responsive to whichthermal energy the detector generates the first signal.

In some embodiments of the present invention, the detector that receiveslight from the light from the light pulse that is not absorbed by thematerial is positioned to receive light from the light pulse that isscattered by the material.

In some embodiments of the present invention, wherein the detector thatreceives energy emitted by the material comprises an acoustic sensor andthe energy emitted by the material responsive to which the detectorgenerates the second signal is acoustic energy.

In some embodiments of the present invention, the detector that receivesenergy emitted by the material comprises a thermal sensor and the energyemitted by the material responsive to which the detector generates thesecond signal is thermal energy.

Optionally, the detector that receives light from the light pulse is thesame detector that receives energy emitted by the material.

Optionally, the detector comprises an acoustic sensor and energy emittedby the material responsive to which the detector generates the secondsignal is a pulse of acoustic energy and wherein the acoustic sensorconverts optical energy from the light pulse incident on the detector toacoustic energy to generate the first signal.

Alternatively the detector optionally comprises a thermal detector andenergy emitted by the material responsive to which the detectorgenerates the second signal is a pulse of thermal energy and wherein thethermal sensor converts optical energy from the light pulse incident onthe detector to thermal energy to generate the first signal.

In some embodiments of the present invention, the path through thematerial is bounded by two surfaces and a portion of the energy in thephotoacoustic pulse emitted by the material repeatedly bounces back andforth between the two surfaces and wherein the processor determines atime period required for energy in the acoustic pulse to make a roundtrip between the surfaces and uses the time period to determine a pathlength for the path and the path length to determine the absorptioncoefficient.

In some embodiments of the present invention, the detector that receivesenergy emitted by the material is positioned so that the path that thelight pulse traverses does not intersect the detector.

BRIEF DESCRIPTION OF FIGURES

Non-limiting examples of embodiments of the present invention aredescribed below with reference to figures attached hereto. In thefigures, identical structures, elements or parts that appear in morethan one figure are generally labeled with a same numeral in all thefigures in which they appear. Dimensions of components and featuresshown in the figures are chosen for convenience and clarity ofpresentation and are not necessarily shown to scale. The figures arelisted below.

FIG. 1 schematically shows a vertical beam photometer determining anabsorption coefficient of a liquid sample, in accordance with anembodiment of the present invention;

FIG. 2 schematically shows a photometer determining an absorptioncoefficient of a solid material, in accordance with an embodiment of thepresent invention;

FIG. 3 schematically shows another photometer determining an absorptioncoefficient of a solid material, in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 schematically shows a vertical beam photometer 20, in accordancewith an embodiment of the present invention, being used to determine anabsorption coefficient for, by way of example, a sample of liquid 22contained in a receptacle 24. Photometer 20 is shown at different timesin the process of determining the absorption coefficient of liquidsample 22 in insets 26 and 28. A graph 30 schematically shows signalsgenerated by photometer 20 as a function of time during the process.Liquid sample 22 has a meniscus 32 at a boundary between the liquidsample and the air. By way of example, in FIG. 1 meniscus 32 is shown asconvex.

Photometer 20 comprises a light source 34, such as a laser, LED or arclamp, an energy detector that is optionally an acoustic detector 36 anda controller 37. Acoustic detector 36 is optionally a piezoelectricdetector. A surface 38 of detector 36 is preferably positioned incontiguous contact with a bottom 40 of receptacle 24, using methodsknown in the art. In inset 26, controller 37 controls light source 34 toilluminate liquid sample 22 with a pulse of light represented by wavyarrows 42.

Light in light pulse 42 that enters liquid 22 is attenuated byabsorption in the liquid as the light pulse propagates in a directiontowards acoustic detector 36. A decreasing number of wavy arrows 42 in adirection from light source 34 to detector 36 schematically indicateattenuation of the light pulse. A portion of the light in light pulse 42is not absorbed by liquid 22, survives travel through liquid sample 22and is incident on surface 38 of acoustic detector 36 as a relativelynarrow pulse of “immediate optical energy”. The pulse of immediateenergy is schematically shown in graph 30 below inset 26 as a pulse 44.Pulse 44 begins at a time t_(o) and has a pulse width substantiallyequal to the pulse width of light pulse 42.

An amount of immediate energy in pulse 44 is proportional to intensityI_(o) of light in light pulse 42 that enters liquid 22. If the amount ofimmediate energy in pulse 44 is represented by “IE” then the immediateenergy can be written IE=β′I_(o) exp (−αD). In the expression for IE, Dis a height of meniscus 32 above detector 36, and α is an absorptioncoefficient of liquid 22 for light in light pulse 42 and β′ is aconstant of proportionality. β′ is substantially equal to the pulsewidth of light pulse 42 times a factor that is an efficiency ofcollection of non-absorbed light from light pulse 42. The efficiencyfactor is a function of the size of detector 36 and scattering of lightin light pulse 42 as the light pulse travels through liquid 22. Theefficiency factor can be calculated using a suitable model of a sampleliquid and shape of receptacle 24 and/or determined experimentally.

Immediate energy pulse 44 causes local heating of detector 36 in aregion of surface 38 of the detector that produces sound waves in thedetector. At a time substantially equal to time t_(o), the detectorgenerates an immediate signal responsive to the sound waves. Theimmediate signal is, via the sound waves, a function of IE and thereforeof the amount of energy from light pulse 42 that is not absorbed byliquid 22. If “IS” represents the immediate signal, I_(o) exp (−αD) maybe written I_(o) exp (−αD)=F(IS) where F represents a processingalgorithm or functional relationship that is usable by controller 37 todetermine I_(o) exp (−αD) from the immediate signal IS.

In some embodiments of the present invention, a functional relationshipbetween IS and I_(o) exp (−αD) is linear. For example, in someembodiments of the present invention, amplitude of the immediate signalor amplitude of the signal integrated over time is a linear function ofthe incident immediate energy. For these embodiments of the presentinvention, if “AIS” represents the “linear” amplitude or time integratedamplitude of the immediate signal then AIS can be written I_(o) exp(−αD)=βAIS. In the expression for AIS, β is a constant ofproportionality, which includes a factor 1/β′. (From the equation abovethat defines β′, immediate energy IE to I_(o) exp (−αD≅IE/β′). β may bedetermined by appropriate calibration of photometer 20.

Light from light pulse 42 that is absorbed in liquid 22 deposits energyin the liquid that generates ultrasound waves by the photoacousticeffect. Sources of the ultrasound waves in liquid 22 are schematicallyshown as “starbursts” 46 in inset 28 of FIG. 1. Since intensity of lightpulse 42 attenuates exponentially as the light pulse travels to detector36, an amount of energy deposited in liquid 22 by the light pulse perunit volume of the liquid decreases exponentially with distance frommeniscus 32. The decrease in deposited energy is schematically indicatedby a decrease in the number of starbursts 46 shown in inset 28 in adirection from meniscus 32 to detector 36.

Ultrasound waves are generated at starbursts 46 following a short timedelay, i.e. a “release delay” after energy is deposited by light pulse42 at locations of the starbursts. Ultrasound waves that originate in astarburst 46 propagate away from the starburst at the speed of soundwith substantially a same intensity in all directions from the starburstand are attenuated as they propagate in accordance with an acousticabsorption coefficient of liquid 22

Some of the ultrasound waves propagate directly from a starburst 46 todetector 36 while some of the ultrasound waves reach detector 36 afterbouncing around in the volume of liquid 22. Energy in the ultrasoundwaves that are incident on detector 36 is “delayed energy” that reachesthe detector following transmission of light pulse 42 through liquid 22.Ultrasound energy that propagates directly from a starburst 46 locatedat a distance “d” from detector 36 reaches the detector after it is“released” from the starburst following a propagation time delay equalto about d/C where C is the speed of sound in the liquid sample.Ultrasound energy from the starburst 46 that does not travel directlyfrom the starburst to detector 36, but instead bounces around in liquid22 (off the walls of the container and the upper surface of the liquid)before reaching the detector, arrives at the detector after it isreleased following a propagation delay that is longer than d/C. The“indirect energy” from the starburst is also attenuated with respect tothe direct energy due to the longer path traveled by the indirect energyin reaching detector 36 and reflective losses. As a result, generally,delayed ultrasound energy reaches detector 36 as a delayed acousticenergy pulse schematically represented by a pulse 48 in graph 30 thatbegins at a time t₁ following a time release delay “Δt_(R)” after timet_(o). Release delay Δt_(R) is a time that elapses from a time at whichenergy is absorbed by a region of liquid 22 to a time at which theregion generates a photoacoustic wave responsive to the absorbed energy.Delayed energy pulse 48 has a maximum at a time about equal to thepropagation time D/C following time t₁ and a pulse width “PW” indicatedin graph 30, that is larger D/C.

The time release delay is on the order of nanoseconds and is muchshorter than the propagation delay, which is on the order ofmicroseconds, that characterizes pulse 48. The time release delay cantherefore generally be ignored in determining an absorption delay (i.e.time release delay plus propagation delay) that characterizes a timefollowing t_(o) at which delayed energy reaches detector 36. In FIG. 1the size of time delay Δt_(R) relative to the size of propagation delayD/C is greatly exaggerated for clarity of presentation. The total amountof ultrasonic energy incident on detector 36 during delayed energy pulse48 is proportional to the total amount of energy absorbed by liquid 22.Let DE represent the total delayed energy incident on detector 36 duringdelayed energy pulse 48. Then DE=γ′[I_(o)(1−exp (−αD))], where theexpression in square brackets is equal to the total amount of energyabsorbed from light pulse 42 by liquid 22 and γ′ is a constant ofproportionality.

In response to delayed energy pulse 48, detector 36 generates a delayedsignal “DS” having a functional relationship to DE and therefore to theamount of energy in light pulse 42 that is absorbed by liquid 22. Letthe functional relationship between DS and the amount of energy in lightpulse 42 that is absorbed by liquid 22 be represented by G(DS) so that[I_(o) (1−exp (−αD))]=G(DS).

In some embodiments of the present invention, the amplitude or timeintegrated amplitude of the delayed signal is a linear function of DE.If the linear amplitude or time integrated amplitude of the delayedsignal DS is represented by ADS, then ADS can be writtenADS≅γ[I_(o)(1−exp (−αD))], where γ is a constant of proportionality thatincludes a factor 1/γ′. (From the definition of γ′, I_(o)(1−exp(−αD))≅DE/γ′).

In accordance with an embodiment of the present invention, a suitableprocessor, (not shown), which may be comprised in controller 37,determines a coefficient of absorption of liquid 22 from an absorptionratio “R”, which is defined by the expression R=G(DS)/F(IS)=[I_(o)(1−exp(−αD))]/[I_(o) exp (−αD)]=(1−exp (−αD))/exp (−αD). It is noted that theratio R is substantially more sensitive to changes in α than is theamount of energy from light pulse 42 that is absorbed by liquid 22 (andtherefore of course also the amount of energy that is not absorbed byliquid 22). The absolute value of the derivative of R with respect to αis greater than the derivative with respect to α of the amount of energyabsorbed from light pulse 42. R is therefore generally a sensitivemeasure of α. For embodiments of the present invention for which theimmediate and delayed signals are “linear functions” of the immediateand delayed energies respectively, R is optionally determined from aratio of the amplitudes or time integrated amplitudes of the immediateand delayed signals, i.e. R=[β(ADS)]/[γ(AIS)].

It is seen from the above equation that R is independent of I_(o). As aresult, a determination of α using R is substantially independent ofintensity of light pulse 42 and variations in output of light source 34.Furthermore, a delay between measurements of immediate and delayedenergy is on the order of a transit time of sound through liquid sample22. The transit time is typically a few microseconds long. During such arelatively short time period, changes in parameters that characterizeand affect operation of components of photometer 20 are expected to besubstantially negligible. As a result, measurements of α determinedusing photometer 20 are substantially immune to drift in theseparameters.

In order to determine α from R a value for the optical path-length D oflight pulse 42 through liquid 22 is required. In some embodiments of thepresent invention D is determined using methods, such as for example amethod described in U.S. Pat. No. 6,188,476 referenced above, availablefrom prior art. In some embodiments of the present invention photometer20 optionally determines a value for D using acoustic energy pulsesreceived by detector 36.

When delayed energy pulse 48 is incident on detector 36, not all of theacoustic energy in the pulse is deposited in the detector. A portion ofthe energy is reflected. The reflected energy propagates towardsmeniscus 32, where at the interface between the meniscus and the air aportion of the reflected energy is again reflected, this time backtowards detector 36. The twice-reflected ultrasonic energy is incidenton detector 36, where again a portion of the incident energy isreflected towards meniscus 32. Acoustic energy from delayed energy pulse48 is thus repeatedly reflected back and forth between meniscus 32 anddetector 36.

The repeatedly reflected energy is incident on detector 36 as a seriesof ultrasonic pulses 50, only two of which are shown, of decreasingamplitude. Pulses 50 have a repetition period “RP” that is about equalto 2D/C, which is a round trip time for sound to travel back and forthbetween detector 36 and meniscus 32. In accordance with an embodiment ofthe present invention, the series of reflected pulses 50 is analyzed bythe processor using methods known in the art to determine a value for D.In some embodiments of the present invention, an ultrasound transducer(not shown) is positioned contiguous with a side wall of receptacle 24.The transducer is used to determine a transit time for sound back andforth between the side wall on which the transducer is positioned andanother side wall of the receptacle. The transit time is used todetermine a value for C.

FIG. 2 schematically shows another photometer 60 in accordance with anembodiment of the present invention. Photometer 60 is similar tophotometer 20 but is not configured as a vertical beam photometer, andis shown by way of example determining an absorption coefficient of asample of a solid material 62.

Photometer 60 operates similarly to photometer 20 and comprisescomponents that are similar to the components comprised in photometer20. When being used to determine an absorption coefficient of a solid,preferably light source 34 is contiguous with and optically coupled to asurface 64 of the solid. Energy detector 36 is preferably in contiguouscontact with a surface 66 of material 62 opposite surface 64 to whichlight source 34 is coupled.

As in the case of photometer 20, controller 37 controls light source 34to transmit a light pulse (not shown) into material 62. Detector 36receives a pulse of immediate energy from light in the light pulse thatis not absorbed by material 62 and generates an immediate signal ISresponsive thereto. Subsequent to receiving a pulse of immediate energy,detector 36 receives a pulse of delayed energy generated by aphotoacoustic effect caused by light in the light pulse that is absorbedby the material and generates a delayed signal DS responsive thereto.The immediate and delayed signals are optionally used to determine anabsorption ratio from which an absorption coefficient of the material isdetermined.

In some embodiments of the present invention, a thickness “D” ofmaterial 62 that separates surfaces 64 and 66 is used to determine anoptical path-length for the light pulse. In some embodiments of thepresent invention, acoustic energy pulses repeatedly reflected back andforth between surfaces 64 and 66 are used to determine a thickness formaterial 62 and thereby an optical path-length for the light pulse.

Whereas in FIG. 2 photometer 60 is shown determining an absorptioncoefficient for a solid material, photometer 60 may be used, inaccordance with an embodiment of the present invention, to determine anabsorption coefficient of a liquid. The liquid is placed in a suitablecuvette which is sandwiched between light source 34 and detector 36similarly to the way in which solid material 62 is sandwiched betweenthe light source and the detector as shown in FIG. 2. A light pulse istransmitted through the cuvette and the liquid it contains to generateimmediate and delayed signals IS and DS that are used to determine anabsorption coefficient for the liquid. To remove effects of the cuvetteon determination of the absorption coefficient of the liquid, a lightpulse is transmitted through the cuvette when it is empty or filled witha liquid, such as water, having an accurately known absorptioncoefficient to provide calibration measurements of immediate and delayedsignals. The calibration measurements are used to correct immediate anddelayed signals generated by detector 36 from which the absorptioncoefficient of the liquid is determined.

FIG. 3 schematically shows another photometer 70, in accordance with anembodiment of the present invention. Photometer 70 is shown being usedto determine an absorption coefficient of a solid material 72 (or liquidin a cuvette).

Photometer 70 operates similarly to photometers 20 and 60. However,unlike photometers 20 and 60, photometer 70 optionally does not comprisean energy detector that is positioned opposite a light source.

Photometer 70 comprises a light source 74 and at least one acousticdetector 76. By way of example photometer 70 is shown with two acousticdetectors 76. Both light source 74 and acoustic detectors 76 arepreferably positioned in contiguous contact with a same surface 78 ofmaterial 72.

As in photometers 20 and 60, to determine an absorption coefficient formaterial 72, light source 74 transmits a pulse of light, represented bywavy arrows 80 that enters the material. However, since detectors 76 arenot positioned opposite light source 74, they do not receive a pulse ofimmediate energy from which to generate an immediate signal from lightin light pulse 80 that completely traverses material 72 directly fromthe light source to the detectors. Instead detectors 76 receive a pulseof immediate energy from light that is back scattered by material 72from light pulse 80 towards the detectors and not absorbed by thematerial. Wavy arrows 82 represent light that is back scattered bymaterial 72 from pulse 80.

In accordance with an embodiment of the present invention, detectors 76generate immediate signals responsive to back scattered light 82.Subsequently, detectors 76 generate delayed signals responsive todelayed energy that reaches the detectors in a pulse of delayed acousticenergy from ultrasound waves generated in a photoacoustic process fromenergy absorbed by material 72 from light pulse 80.

The immediate and delayed signals are processed, in accordance with anembodiment of the present invention, to determine an absorption ratio,which absorption ratio is used together with an optical path-length forlight pulse 80 in material 72 to determine an absorption coefficient forthe material. In some embodiments of the present invention, the opticalpath-length is determined from known dimensions of material 72. In someembodiments of the present invention photometer 70 is operated similarlyto detectors 20 and 60 and multiple reflections of ultrasound energyfrom the delayed acoustic pulse are processed to determine thickness ofmaterial 72 and thereby an optical path-length for light pulse 80.

A photometer, in accordance with an embodiment of the present invention,similar to photometer 70 is particularly advantageous when it is notpossible or advantageous to sandwich a sample of a material between alight source and an energy detector in order to determine an absorptioncoefficient for the material.

Furthermore, in some embodiments of the present invention, for amaterial having a thickness substantially greater than an inverse of anabsorption coefficient of the material, photometer 70 operates todetermine the absorption coefficient without need to determine anoptical path-length in the material for light that is used to determinethe absorption coefficient. For example, for such a situation, using avery simplified model and assuming single scattering, an amount ofimmediate energy IE incident on detectors 76 from a light pulse 80 ofpulse length “τ” and initial intensity I_(o) may be writtenIE = τ∫₀^(∞)∫_(2π)^(4π)I_(o)ɛ(x, Ω)σ(Ω)exp (−2α  x)  𝕕x  𝕕Ω.In the expression for IE, x represents depth into the material, σ(Ω) isan elastic scattering cross section for light as a function of solidangle and ε(x,Ω) is a “geometrical” collection efficiency of detectors76 for light back scattered into a solid angle Ω from a depth x in thematerial. The factor 2 appears in the argument of the exponentialfunction to account, approximately, for attenuation of light that isback scattered to detectors 76. (A path-length of light back scatteredto detectors 76 from a depth x is approximated in the above expressionfor IE as equal to 2x.) Integration over solid angle is over the “backsolid angles”, from solid angle 2π to solid angle 4π, and integrationover depth of the material is from 0 to ∞. Integration is performed overthe back solid angles because light reaching detectors 76 is backscattered light. Integration over depth x is from 0 to infinity becauseit is assumed that thickness of the material is much greater than anabsorption length, 1/α, of the material. In practice, generally asubstantially more complicated model and/or numerical methods such asMonte-Carlo may be used to determine IE.

A similar expression for delayed energy DE that reaches detectors 76 maybe writtenDE = τ  ∫₀^(∞)∫_(2π)^(4π)I_(o)ɛ(x, Ω)ρ(α  exp (−α  x))  𝕕x  𝕕Ω.In the expression for DE, τ(α exp (−αx)) is an amount of energy absorbedfrom light pulse 80 per unit volume of the material at a depth x, and ρis a proportionality constant that relates the amount of absorbed energyto intensity of a photoacoustic wave generated in a volume of thematerial that absorbs the energy. (For simplicity it is assumed that ρis a constant independent of the amount of absorbed energy.)

From the expressions for IE and DE it is seen that IE and DE areindependent of path-length of the light pulse in the material. Thegeometric collection efficiency can be determined from a proper modelingof the geometry of photometer 70 and an assumption regarding scatteringof light in the light pulse as a function of depth traveled in thematerial.

However, to determine absorption coefficient α from the aboveexpressions for IE and DE the elastic scattering cross-section forlight, σ(Ω), and the photoacoustic coupling coefficient, ρ, must beknown. In some embodiments of the present invention, σ(Ω) and ρ areestimated from cross-sections and photoacoustic coupling constants thatare known for materials similar to the material for which an absorptioncoefficient is being determined.

In the above discussion, energy detectors used to detect immediateenergy IE and delayed energy DE have been assumed to be acousticdetectors. Photometers, in accordance with some embodiments of thepresent invention, comprise in place of acoustic detectors, energydetectors that are thermal detectors that generate signals responsive tothermal energy that they receive. Components and configurations ofphotometers, in accordance with embodiments of the present invention,that comprise thermal detectors are similar to configurations ofphotometers that comprise acoustic detectors, in accordance withembodiments of the present invention, with the acoustic detectorsreplaced with thermal detectors. A “thermal photometer”, in accordancewith an embodiment of the present invention operates similarly to themanner in which a corresponding “acoustic photometer” operates.

When a light pulse from a light source in a thermal photometer istransmitted through a material for which an absorption coefficient is tobe determined, at least some of the light in the light pulse that is notabsorbed by the material is incident on a thermal detector that thephotometer comprises. The incident light heats the thermal detector,transmitting immediate energy to the thermal detector in the form ofthermal energy. The thermal detector generates an immediate signal ISresponsive to the immediate thermal energy. Light from the light pulsethat is absorbed by the material heats the material. Thermal energy fromregions of the material heated by the light pulse propagates away fromthe region by convection and is incident on the thermal detector asdelayed energy, responsive to which the thermal detector generates adelayed signal DS. In accordance with an embodiment of the presentinvention, the immediate and delayed signals provided by the thermaldetector are used to determine an absorption ratio from which anabsorption coefficient of the material is determined.

It is to be noted that whereas in the above examples of photometers inaccordance with embodiments of the present invention, a same detector isused to sense immediate energy and delayed energy, in some embodimentsof the present invention different detectors are used to sense immediateand delayed energy. For example, a first detector that senses immediateenergy might be positioned, as shown in FIGS. 1-3, i.e. opposite oradjacent to a light source that radiates a light pulse into a materialwhose absorption coefficient is being measured. A second detector thatsenses delayed energy might be located on a surface of the material thatis substantially parallel to a direction along which the light sourceradiates the light pulse. (It is noted that delayed energy is generallyemitted substantially isotropically by a region of the material thatabsorbs energy from a light pulse transmitted into the material. As aresult, a position for a second detector that senses delayed energyother than positions shown in FIGS. 1-3, for example as noted above on asurface parallel to a direction along which the light pulse propagates,is possible and can be advantageous.)

Furthermore, by using different detectors for sensing immediate anddelayed energy, in accordance with an embodiment of the presentinvention, detectors used to sense immediate energy can be optimized tosense optical energy (i.e. suitable optical detectors), whereasdetectors used to sense delayed energy can be optimized to detect aparticular desired form of delayed energy, e.g. acoustic or thermal.

It is further noted that in some embodiments of the present invention,delayed energy as well as immediate energy may be optical energy. Forexample, optical energy absorbed from a light pulse by a sample whoseabsorption coefficient is being measured, in accordance with anembodiment of the present invention, may cause the sample material toluminesce following a release delay. The luminesced light is sensed andused to determine the amount of delayed energy. Generally, theluminesced light is characterized by a wavelength that is different thanthe wavelength of the light that characterizes the light pulse fromwhich the optical energy is absorbed. As a result, light proportional toimmediate energy may be distinguished, in accordance with an embodimentof the present invention, from luminesced light proportional to delayedenergy not only by temporal separation (i.e. by absorption delay) butalso by difference in wavelength.

In the description and claims of the present application, each of theverbs, “comprise,” “include” and “have”, and conjugates thereof, areused to indicate that the object or objects of the verb are notnecessarily a complete listing of members, components, elements or partsof the subject or subjects of the verb.

The present invention has been described using detailed descriptions ofembodiments thereof that are provided by way of example and are notintended to limit the scope of the invention. The described embodimentscomprise different features, not all of which are required in allembodiments of the invention. Some embodiments of the present inventionutilize only some of the features or possible combinations of thefeatures. Variations of embodiments of the present invention that aredescribed and embodiments of the present invention comprising differentcombinations of features noted in the described embodiments will occurto persons of the art. The scope of the invention is limited only by thefollowing claims.

1. A method of determining a portion of light at a given wavelengthwhich is incident on a material that is absorbed by the material, themethod comprising: transmitting a pulse of light at the given wavelengthso that the pulse traverses a path through the material; generating afirst signal responsive to light in the light pulse that is not absorbedbut is scattered by the material relative to a direction of propagationof the at least one light pulse; generating a second signal responsiveto energy that the material emits responsive to a portion of the lightfrom the light pulse that is absorbed by the material as the light pulsetraverses the path; and using the first and second signals to determinethe absorbed portion.
 2. A method according to claim 1 and comprisingdetermining a path length for the path that the light pulse traversesand using the determined path length and the absorbed portion todetermine an absorption coefficient of the material for the givenwavelength.
 3. A method according to claim 2 wherein the energy that thematerial emits comprises a pulse of acoustic energy generated in thematerial by a photoacoustic effect and generating the second signalcomprises sensing the acoustic energy and generating a signal responsivethereto.
 4. A method according to claim 3 wherein the path through thematerial is bounded by two surfaces and a portion of the energy in thephotoacoustic pulse emitted by the material repeatedly bounces back andforth between the two surfaces and determining a path length for thepath comprises determining a time period it takes for energy in theacoustic pulse to make a round trip between the surfaces and using thetime period to determine the path length.
 5. A method according to claim1 wherein the energy that the material emits comprises a pulse ofacoustic energy generated in the material by a photoacoustic effect andgenerating the second signal comprises sensing the acoustic energy andgenerating a signal responsive thereto.
 6. A method according to claim 1wherein the energy that the material emits comprises thermal energy andgenerating the second signal comprises sensing the thermal energy andgenerating a signal responsive thereto.
 7. A method according to claim 1wherein the energy that the material emits comprises optical energyluminesced by the material and generating the second signal comprisessensing the luminesced light and generating a signal responsive thereto.8. A method according to claim 1 wherein generating a first signalcomprises sensing optical energy in the non-absorbed light, transducingthe sensed energy to acoustic energy and generating a signal responsiveto the acoustic energy.
 9. A method according to claim 1 whereingenerating a first signal comprises sensing optical energy in thenon-absorbed light, transducing optical energy in the non-absorbed lightto thermal energy and generating a signal responsive to the thermalenergy.
 10. A method according to claim 1 comprising sensing energyoriginating in the light pulse as a function of time followingtransmission of the light pulse through the material and generating thefirst signal comprises generating the first signal responsive to energysensed within a time period after transmission of the light pulse thatis less than or equal to about twice a transit time of light from thelight pulse over the path.
 11. A method according to claim 10 whereingenerating the second signal comprises generating a second signalresponsive to energy sensed at time following the light pulsetransmission time that is substantially later than the transit time. 12.A method according to claim 1 wherein using the first and second signalsto determine the absorbed portion comprises: using the first signal toprovide an indication of energy in the light pulse that is not absorbedby the material; using the second signal to provide an indication ofenergy in the light pulse that is absorbed by the material; and usingthe indicated energies to determine the absorbed portion.
 13. A methodaccording to claim 12 wherein using the indicated energies to determinethe absorbed portion comprises determining a quotient between theindicated energies.
 14. A method according to claim 1 wherein generatingthe first and second signals comprises using a same detector to sensethe non-absorbed light and the energy that the material emits. 15.Apparatus for determining an absorption coefficient of a material forlight of a given wavelength comprising: a light source that transmits apulse of light at the given wavelength that traverses a path through thematerial; a detector that receives light from the light pulse that isnot absorbed but is scattered by the material relative to a direction ofpropagation of the at least one light pulse and generates a first signalresponsive thereto; a detector that receives energy emitted by thematerial responsive to light from the light pulse that is absorbed bythe material and generates a second signal responsive to the receivedenergy; and a processor that receives the first and second signals anduses the signals to determine the absorption coefficient.
 16. Apparatusaccording to claim 15 wherein the detector that receives light from thelight pulse comprises an acoustic sensor that converts optical energyfrom the light pulse incident on the detector to acoustic energyresponsive to which acoustic energy the detector generates the firstsignal.
 17. Apparatus according to claim 15 wherein the detector thatreceives light from the light pulse is a thermal sensor that convertsoptical energy from the light pulse incident on the detector to thermalenergy, responsive to which thermal energy the detector generates thefirst signal.
 18. Apparatus according to claim 15 wherein the detectorthat receives energy emitted by the material comprises an acousticsensor and the energy emitted by the material responsive to which thedetector generates the second signal is acoustic energy.
 19. Apparatusaccording to claim 15 wherein the detector that receives energy emittedby the material comprises a thermal sensor and the energy emitted by thematerial responsive to which the detector generates the second signal isthermal energy.
 20. Apparatus according to claim 15 wherein the detectorthat receives light from the light pulse is the same detector thatreceives energy emitted by the material.
 21. Apparatus according toclaim 20 wherein the detector comprises an acoustic sensor and energyemitted by the material responsive to which the detector generates thesecond signal is a pulse of acoustic energy and wherein the acousticsensor converts optical energy from the light pulse incident on thedetector to acoustic energy to generate the first signal.
 22. Apparatusaccording to claim 20 wherein the detector comprises a thermal detectorand energy emitted by the material responsive to which the detectorgenerates the second signal is a pulse of thermal energy and wherein thethermal sensor converts optical energy from the light pulse incident onthe detector to thermal energy to generate the first signal. 23.Apparatus according to claim 18 wherein the path through the material isbounded by two surfaces and a portion of the energy in the photoacousticpulse emitted by the material repeatedly bounces back and forth betweenthe two surfaces and wherein the processor determines a time periodrequired for energy in the acoustic pulse to make a round trip betweenthe surfaces and uses the time period to determine a path length for thepath and the path length to determine the absorption coefficient. 24.Apparatus according to claim 15 wherein the detector that receivesenergy emitted by the material is positioned so that the path that thelight pulse traverses does not intersect the detector.
 25. Apparatusaccording to claim 21 wherein the path through the material is boundedby two surfaces and a portion of the energy in the photoacoustic pulseemitted by the material repeatedly bounces back and forth between thetwo surfaces and wherein the processor determines a time period requiredfor energy in the acoustic pulse to make a round trip between thesurfaces and uses the time period to determine a path length for thepath and the path length to determine the absorption coefficient.